Method of cleaning permeable formations



LAN-UV vw United States Patent 3,160,206 METHOD OF CLEANING PERMEABLE FORMATIONS Robert M. Jorda, Houston, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed June 7, 1961, Ser. No. 115,299 8 Claims. (Cl. 166-30) This invention relates to a method of cleaningthe permeable formations of oil, gas and water wells and pertains more particularly to cleaning a long sand interval traversed by a Well from which fluids are produced or into which fluids are injected. The invention further relates to a method of treating a well by forcing a selected liquid to enter substantially all permeable portions of an interval of formations having varying degrees of initial permeability and accomplishing this without permanently reducing the permeability of any of the formations in the interval being treated.

During the drilling of oil and gas wells a drilling fluid or mud is circulated in the well and forms a sheath on the borehole wall which prevents loss of fluid into the formation. After well drilling operations have been completed, this mud sheath is normally washed off the wall of the borehole by circulating clean fluid in the well. At times particles of a mud work their way into the formation sufficiently to plug portions of the well thus reducing the area of the well wall from which production fluid in the formation may flow. It is therefore desirable to remove all of the mud sheath. One method of doing this is to acidize the well and bring a cleaning acid into contact with the face of the permeable formation to be cleaned after the major portion of the mud has been washed therefrom.

Acidization has been effectively employed to clean and open a relatively small interval, say feet of permeable formation, due to the fact that a large charge of acid pumped down to this small interval of formation treats all of the formation to some degree. However, for a well having a long interval of permeable formation to be cleaned, say from 10 to 500 feet, it may readily be seen that when a large charge of cleaning acid or other cleaning fluid is pumped into the well and positioned adjacent the interval or formation to be cleaned, the major portion, if not all, of the acid or cleaning fluid will be injected into the most permeable portion of the formation, thus opening this portion more and making it more permeable while the less permeable portions of the interval will not be treated at all.

The tendency for a cleaning fluid to be channeled to the most permeable portion of the formation interval and being lost therein, holds true for whatever type of cleaning fluid is being employed whether it be a hydrocarbon or non-hydrocarbon fluid. In order to prevent loss of the major portion of the cleaning fluid volume into the most permeable portion of a long interval of permeable sand to be cleaned in a well, it is generally necessary to employ a pipe string having a straddle packer or a pair of packers positioned in closely spaced relationship at the lower end of a pipe string. with fluid discharge ports between the packers. Thus, by setting the packers a foot apart and sealing them against a well Wall, it is possible to acidize only a foot of the formation. Subsequently, the acidization step could be repeated many times with the packers being unset and then moved axially in the borehole where they would be set again against the borehole wall.

However, even the use of a straddle packer in the above-described manner is not possible where the well is provided with an uncemented liner opposite the permeable formation through which cleaning fluid is to be injected. It is apparent that even if a straddle packer were used, the

gaass 3,160,206 Patented Dec. 8, 1964 "ice cleaning fluid would be free to move up or down the well outside the perforated liner to the most permeable portion of the interval and be lost therein.

A formation interval into which a well is opened can undergo a reduction in permeability due to numerous things other than contamination by the particles in a drilling fluid. The formations in such an interval may be plugged by the precipitation of carbonates, asphalts, paraflin, salt, etc., as well as by the swelling of clay particles which were contacted by fresh water or other fluids, such as certain mud filtrates, within the formations, etc. In addition, there are various situations in which it is desirable to inject a treatment liquid as uniformly as possible into all of the permeable formations in an interval of nonuniformly permeable formations which have undergone no significant reduction in their natural permeability. For example, in intervals of unconsolidated formations, it may be deisrable to so inject an oil which can be thermally coked to form a coke residue which consolidates the formation. In all such situations, the generally similar problems are involved in forcing the injected liquid to enter the zones of lower permeabilities as well as the zones of higher permeabilities.

It is therefore a primary object of the present invention to provide a method for cleaning all portions of a formation extending over a long well interval without loss of cleaning fluid to the most permeable portion of the formation.

Another object of the present invention is to provide a method for cleaning the permeable formations in oil, gas and water wells to increase the permeability of a long section of well formation into which a fluid is to be injected, or from which formation fluid is to be produced.

A further object of the present invention is to provide a method for successively increasing the permeability of portions of a formation and subsequently successively plugging each portion of the formation of increased permeability until all of the portions of a long interval of permeable formation traversed by a well have been treated.

These and other objects of this invention will be understood from the following detailed description.

The invention provides a method for injecting a liquid into an interval of earth formations in a manner such that the injected liquid penetrates into substantially all portions of the formation and none of the formations are fractured or permanently plugged, where, at the time of the treatment, different portions of the formations in the interval have different effective permeabilities. The method comprises injecting into the interval of earth formations a sequence of slugs of the selected liquid which are separated by slugs of a plugging liquid having a viscosity which is high relative to that of the selected liquid and is capable of being converted, in situ within the formation, to a relatively low viscosity. The volume and viscosity of the slugs of the plugging agent are selected so that an increase in the pumping pressure, an increase to a pressure less than the fracturing pressure, causes substantially all of each slug of the plugging material to be injected into portions of the formation into which the selected liquid has been injected. The injection of the sequence of slugs is continued until the selected liquid has been forced into substantially all of the permeable portions in the interval. Subsequently, the viscosity of the plugging material is reduced in situ within the formation.

The present process is particularly effective in treating an interval of formations with a cleaning liquid which restores the original effective permeability of each portion that it penetrates, e.g., an acid or alkali which dissolves-the mud or other materials that plug the pores of the formation or an organic solvent which dissolves such plugs of asphalt or paraflin. In this type of operation, the injection of each slug of the cleaning liquid tends to make a predictable restoration of the original permeability and thus, in effect, tends to open a portion of known volume within the formation. The viscosity and volume of the plugging slugs can be selected so that most, if not all, of the plugging material enters the portion of the formation into which the preceding slug of cleaning liquid has been injected and most, if not all, of the next slug of cleaning fluid is forced to enter an untreated portion of lesser permeability.

In the following description of the present invention, a permeable formation traversed by a well may be considered to be either the production zone of a well from which oil, gas or water is produced, or the injection zone of a well into which a fluid may be injected for various purposes, such as for disposing of the fluid, for pressuring up the formation to maintain the pressure therein, for flooding purposes in secondary recovery methods, etc. It is recognized that a permeable formation traversed by a well bore is seldom of uniform permeability throughout the length of the borehole section. Variations in permeability may be due to the natural characteristics of the earth formation or sand being considered, or may be due to uneven plugging of the permeable formation that has taken place during drilling operations, during the production of fluid from the formation, or during the injection of fluid into the formation.

The present method concerns a method of treating an earth formation of varying permeability, which formation is penetrated by a Well and is in communication therewith. In the event that the well is cased it would be necessary to provide fluid communication through the well casing at a point adjacent the permeable formation to be cleaned prior to carrying out the method of the present invention. To clean a long interval of a permeable formation traversed by a well, a predetermined volume of cleaning fluid is pumped down the well and into contact with the formation, if desired suificient pressure may be provided to assure the cleaning fluid being injected into the formation. If desired, well packers may be used above and below the long interval of formations to be cleaned but usually these are not necessary unless the well being treated is a multiple completion well in which it is desirable to isolate the interval being treated.

The injected cleaning fluid seeks out and enters the most permeable portion of the formation. A predetermined volume of a formation plugging material is then pumped down the well behind the cleaning fluid to follow the cleaning fluid into the most permeable formation and to seal this portion of the formation against further entry of fluid. These two steps are successively repeated many times, first pumping down a volume of cleaning fluid and then sealing the portion of a formation cleaned by pumping down a volume of sealing fluid to contact the formation.

The cleaning fluid used would be of a type to increase the permeability of the formation to be cleaned and the general type of cleaning fluid used would depend on the material to be cleaned from the formation. For example, a drilling mud skin that plugs the permeable zone of the formation may need an acid to remove it. On the other hand if contaminating material on the porous sand formation is a heavy hydrocarbon, a hydrocarbon solvent such as any light hydrocarbon, for example naphthalene, would be used as a cleaning agent.

The plugging material that is pumped down the well in alternate slugs between the slugs of cleaning fluid is of the general type that will effectively plug the formation temporarily and breaks down to a free-flowing liquid after being exposed for a predetermined time to well conditions of temperature. pressure, formation fluids, etc., at the level of the portion of formation that is being cleaned. A liquid hydrocarbon, such as dieselbil, in which a mixture of fatty acids, such as the tall oil acids,

have been dissolved and converted to their soaps is one such suitable composition which will break down when exposed to well temperatures. An especially good plugging material of this type is a mixture of diesel oil and 15 percent hydrochloric acid with a small amount of an emulsifying agent, preferably an organic emulsifier such as 2 percent of a non-ionic polyamide. The other components are present roughly in the ratio of 30 percent diesel oil to 70 percent acid, with the percentages of the components being varied so that the emulsion formed therefrom will stay emulsified and effectively plug a permeable formation at bottomhole well temperatures for at least 8 hours. After the plugging material breaks down to a free-flowing fluid, it is removed from the well either by artificial lift or natural drive by forcing it into and dispersing it within the formation.

The number of portion or slugs of cleaning fluid and plugging material to be pumped down a well alternately for carrying out the method of the present invention depends mainly upon the area of the porous formation to be cleaned. The injection of a predetermined volume of cleaning fluid followed by a predetermined volume of plugging material may be known as a stage. The number of stages to be employed would depend upon the area of porous formation to be treated. From five to fifty or more stages have been employed. The volume of cleaning is adjusted for type of formation, considering its permeability and porosity.

The pressures required to cause the successive stages to enter the formations in the interval being treated are predictable and deviations from the predicted pressure behavior can be utilized in adjusting the stages to improve the treatment. For example, Where an interval of relatively uniformly permeable formations is plugged by a skin having a relatively low and uniform permeability, the cleaning liquid is selected to provide a suitable restoration of the original permeability of the formations. The volumes selected for the slugs of cleaning liquid are those expected to cause the liquid to contact a significant area of the skin as it penetrates farther into the formation. The slugs of plugging liquid are selected to have (1) an effective viscosity such that they will enter the portions of the formation of restored permeability under pressures above those required for the injection of the cleaning liquid but below the fracturing pressure and (2) a volume such that substantially all of the plugging liquid will penetrate into the area from which the skin has been removed. The predictable pressure behavior is a series of highs and lows of approximately constant magnitude. Abnormal highs would indicate a failure to dispose of all of each plugging slug in the portion cleaned by the preceding cleaning slug.

Where the well is shallow and the tubing string contains only a few of the selected liquid and the plugging liquid stages, the ratio of the volumes of cleaning and plugging slugs can be adjusted by simply changing the volumes metered into the string. Where the well is deep and an immediate change is needed, the stages in the borehole and the tubing string can be backflushed and replaced by slugs of the proper relative sizes. The backflushing can readily be accomplished, for example, by opening the top of the tubing string, releasing the upper packer if one is used, and pumping a displacing fluid down the annulus. The train of slugs of a suitable size can then be spotted adjacent to the interval by reversing the backflushing procedure.

In injecting cleaning liquids or other liquids of relatively low viscosity into an interval in which the original or effective permeabilities are nonuniform, the desirable pressure curve would take the form of an ascending sawtoothed curve. The plugging liquid can suitably have a viscosity such that it penetrates the most permeable zone when the injection pressure has undergone only a nominal increase, in which case several stages would be forced into the permeable zone before the injection pressure had reached a magnitude suflicient to force the relatively low viscosity liquid to enter the zone of the next lower permeability. Alternatively, the viscosity of the plugging liquid can suitably be one requiring a pronounced increase in the injection pressure to cause it to penetrate into the most permeable zone, in which case it would be necessary to use a plugging liquid of a lower viscosity in order to force the relatively low viscosity liquid to penetrate all portion of the less permeable zones.

Where it is desirable to cause a selected liquid to penetrate relatively deeply into the formations, the volume of the slugs of the selected liquid should be several times greater than those of the plugging liquids. The suitable viscosities for the plugging liquid and the desirable pressure curves would be influenced in the manner described above by any lack of uniformity in the formation permeabilities.

The following examples are given to illustrate some uses to which the method of the present invention has been carried out to clean long well intervals of permeable formation which have been contaminated.

In one field, treatment of 300 foot intervals of permeable formation at 12,000 foot level in wells with ordinary large single stage acid application failed to give any appreciable results. The permeable formation was plugged in this case with a mud skin, and the wells were being used as gas injection wells. Two of the wells were treated in accordance with the present invention when using hydrofluoric acid as the cleaning agent, with a plastering fluid made up of approximately 30 percent oil and approximately 70 percent of 15 percent strength hydrochloric acid which was emulsified using two percent of a polyamide as an emulsifier. Five barrel slugs of hydrofluoric acid were used as the cleaning fluid and two barrel slugs of the emulsion were used as the plugging fluid. The slugs were pumped alternately down the well and 50 slugs of each fluid were used for the 300 foot interval of 8-inch hole in which a 5 and /2 inch liner was positioned. The composition of the plugging agent was adjusted so that it broke to a free-flowing fluid after 24 hours at well temperature. The injectivity of the two wells increased from 20 million cubic feet of gas per day at an injection pressure of 6000 p.s.i. to 120 million cubic feet at an injection pressure of 6000 p.s.i.

The same cleaning and plugging fluids used in the example above were used to clean a 50 foot permeable interval in a water injection well at a 5,000 depth. The permeability formation was contaminated with a skin which had been formed during the inadvertent injection of dirty water into the formation, which skin was made up principally of calcium carbonate, sand and iron sulfide. A customary single-stage large-volume acid treatment gave no measureable improvement in the well. Subsequently, the well was treated in accordance with the method of the present invention using ten slugs each of the cleaning fluid and the plugging fluid. After the temporary plugging material had turned to fluid, the injectivity of the permeable interval increased from 600 barrels a day at 500 p.s.i. injection pressure to 6,000 barrels a day at a vacuum.

As another example, a gas injection well had a 300 foot permeable interval contaminated at the 12,000 foot level with compressor oil which had been plastered on the sand face when it has become entrained in the gas stream being injected into the formation. The method of the present invention was carried out using naphthalene as the cleaning agent and the same diesel oil-acid emulsion as the plug. Fifty stages were used with five barrels of cleaning fluid and two barrels of plugging fluid in each stage. The injection rate of the well increased from 25 to 90 million cubic feet at 6000 p.s.i injection pressure a day after cleaning.

In another field a gas producing well had a 150 foot interval of a dolomite limestone matrix at a depth of 12,000 feet to be cleaned. Fifteen percent hydrochloric acid was used as a cleaning fluid and the plugging fluid was an emulsion of 88 percent of 15 percent hydrochloric acid plus 10 percent kerosene plus 2 percent non-ionic polyamide emulsifier. Three thousand gallons of cleaning fluid and 500 gallons of plugging emulsion were used in each stage. Six stages were used to treat the formation in accordance with the present invention and the gas production rate went from 6 million cubic feet per day to 16 million cubic feet per day. Previously, a 100,000 gallon treatment of 15 percent hydrochloric acid did not give any measureable improvement in the production rate as all of the acid went out a porous formation.

In another field normal fracturing and acidization treatments did not increase gas production from a foot interval between 11 and 12,000 feet. It was desired to form fractures in multiple zones. The cleaning agent employed was a combination of 15 percent acid and Walnut shells as a propping agent. The plugging fluid was made up of equal volumes of 15 percent hydrochloric acid and naptha base plugging material. 500 gallons each of the cleaning fluid and plugging fluid were used in each stage and three stges were used to treat the well. The gas production increased from 10,000,000 to 20,000,000 cubic feet per day.

I claim as my invention:

1. A method of treating an earth formation penetrated by a well and in communication therewith for increasing the permeability of said formation adjacent said well, said method comprising the steps of pumping a predetermined volume of a cleaning fluid down the well and into contact with said formation, injecting said cleaning fluid into a portion of said formation, pumping a predetermined volume of a formation-plugging material down the well behind the cleaning fluid and contacting the formation with it to seal the most permeable Zone thereof, and subsequently repeating said steps of successively pumping alternate volumes of cleaning fluid and sealing material down the well and into contact with unsealed portions of said formation, each of said volumes being a minor amount of the volume used in a one-stage treatment, said plugging material being of a composition that breaks down to a free-flowing fluid after being exposed for a predetermined time to the well conditions at the level of the portion of formation being treated.

2. A method of treating an earth formation penetrated by a well and in communication therewith for increasing the permeability of said formation adjacent said well, said method comprising the steps of pumping a predetermined volume of a cleaning fluid down the well and into contact with said formation, injecting said cleaning fluid into a portion of said formation, pumping a predetermined volume of a formation-plugging material down the well behind the cleaning fluid and contacting the formation with it to seal the most permeable zone thereof, subsequently repeating said steps of successively pumping alternate volumes of cleaning fluid and sealing material down the well and into contact with unsealed portions of said formation, each of said volumes being a minor amount of the volume used in a onestage treatment, said cleaning fluid being of a type to increase the permeability of the formation forming the well wall, said plugging material being of a composition that breaks down to a free-flowing fluid after being exposed for a predetermined time to the well conditions at the level of the portion of formation being treated, and waiting for a time sufficient for the plugging material to turn to a free-flowing fluid.

3. A method of treating an earth formation penetrated by a well and in communication therewith for increasing the permeability of said formation adjacent said well, said method comprising the steps of providing fluid communication through a Well casing within a well adjacent the zone of the formation to be treated, pumping a predetermined volume of a cleaning fluid down the well and into contact with said formation, injecting said cleaning fluid into a portion of said formation, pumping a predetermined volume of a formation-plugging material down the well behind the cleaning fluid and contacting the formation with it to seal the mose permeable zone thereof, subsequently repeating said steps of successively pumping alternate volumes of cleaning fluid and sealing material down the well and into contact with unsealed portions of said formation, each of said volumes being a minor amount of the volume used in a one-stage treatment, said cleaning fluid being of a type to increase the permeability of the formation forming the well wall, said plugging material being of a composition that breaks down to a free-flowing fluid after being exposed for a predetermined time to the well conditions of temperature and pressure at the level of the portion of formation being treated, waiting for a time sufficient for the plugging material to turn to a free-flowing fluid, and removing said freeflowing liquid from said well by applying fluid pressure to force it in said formation.

4. A method of treating an earth formation penetrated by a well and in communication therewith for increasing the permeability of said formation adjacent said well, said method comprising the steps of providing fluid communication through a Well casing within a Well adjacent the zone of the formation to be treated, pumping a'predetermined volume of a cleaning fluid down the well and into contact with said formation, injecting said cleaning fluid into a portion of said formation, pumping a predetermined volume of a formation-plugging material down the well behind the cleaning fluid and contacting the formation with it to seal the most permeable zone thereof, subsequently repeating from five to one hundred times said steps of successively pumping alternate volumes of cleaning fluid and sealing material down the well and into contact with unsealed portions of said formation, each of said volumes being a minor amount of the volume used in'a one-stage treatment, said cleaning fluid being of a type to increase the permeability of the formation forming the well wall, said plugging material being of a composition that breaks down to a free-flowing fluid after being exposed for a predetermined time to the Well conditions of temperature and pressure at the level of the portion of formation being treated, waiting for a time suflicient for the plugging material to turn to a free-flowing fluid, and displacing said free-flowing liquid from said well by applying fluid pressure to force it into said formation and disperse it.

5. The method of claim 1 wherein said cleaning fluid is a hydrocarbon solvent.

6. The method of claim 1 wherein said cleaning fluid is an acid 7. A method of treating an earth formation penetrated by a well and in communication therewith, said method comprising the steps of pumping a predetermined volume of a formation treating liquid down the Well and into contact with said formation, injecting said formation treating liquid into a portion of said formation,- pumping a predetermined volume of a liquid formation-plugging material down the Well behind the selected liquid and contacting the formation with it to seal the most permeable zone thereof, and subsequently repeating said steps of successively pumping alternate volumes of treating liquid and liquid-plugging material down the well and into contact with unsealed portions of said formation, each of said Volumes being a minor amount of the volume used in a one-stage treatment, said liquid-plugging material being of a composition adapted to break down to a freeflowing fluid.

8. A method of treating an earth formation penetrated by a Well and in communication therewith, said method comprising the steps of pumping alternate volumes of a relatively low-viscosity formation-treating liquid and a relatively high-viscosity liquid-formation-plugging material down the well and into contact with said formation, injecting a volume of said treating liquid into a portion of said formation, increasing the pumping pressure to a pressure less than a pressure which fractures said formation to pump a volume of said plugging material into the formation to seal the most permeable zone thereof, subsequently repeating said step of adjusting the pumping pressure as required to pump alternate volumes of treatting liquid and plugging material into unsealed portions of said formation, each of said volumes being a minor amount of the volume used in a one-stage treatment, said treating liquid being of a type to increase the permeability of the formation forming the Well wall, said plugging material being of a composition that breaks down to a freeflowing fluid after being exposed for a predetermined time to the well conditionsat the level of the portion of formation being treated, and waiting for a time suflicient for the plugging material to turn to a free-flowing fluid.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,595 Lerch et al Dec. 6, 1938 2,207,759 Reimers July 16, 1940 2,699,832 Allen Jan 18, 1955 2,734,861 Scott et al Feb. 14, 1956 2,804,145 Holbrook Aug. 27, 1957 

1. A METHOD OF TREATING AN EARTH FORMATION PENETRATED BY A WELL AND IN COMMUNICATION THEREWITH FOR INCREASING THE PERMEABILITY OF SAID FORMATION ADJACENT SAID WELL, SAID METHOD COMPRISING THE STEPS OF PUMPING A PREDETERMINED VOLUME OF A CLEANING FLUID DOWN THE WELL AND INTO CONTACT WITH SAID FORMATION, INJECTING SAID CLEANING FLUID INTO A PORTION OF SAID FORMATION, PUMPING A PREDETERMINED VOLUME OF A FORMATION-PLUGGING MATERIAL DOWN THE WELL BEHIND THE CLEANING FLUID AND CONTACTING THE FORMATION WITH IT TO SEAL THE MOST PERMEABLE ZONE THEREOF, AND SUBSEQUENTLY REPEATING SAID STEPS OF SUCCESSIVELY PUMPING ALTERNATE VOLUMES OF CLEANING FLUID AN SEALING MATERIAL DOWN THE WELL AND INTO CONTACT WITH UNSEALED PORTIONS OF SAID FORMATION, EACH OF SAID VOLUMES BEING A MINOR AMOUNT OF THE VOLUME USED IN A ONE-STAGE TREATMENT, SAID PLUGGING MATERIAL BEING OF A COMPOSITION THAT BREAKS DOWN TO A FREE-FLOWING FLUID AFTER BEING EXPOSED FOR A PREDETERMINED TIME TO THE WELL CONDITIONS AT THE LEVEL OF THE PORTION OF FORMATION BEING TREATED. 